Gas Pipework Installation: Materials, Sizing, Burial and Pressure Testing

Summary

Gas pipework installation is one of the most tightly regulated areas of building services. A gas leak or poorly installed pipe joint is a potential source of explosion, fire, or carbon monoxide poisoning. Every aspect of the installation — from material selection and pipe sizing through to burial depth and pressure testing — is governed by specific standards and Gas Safe registration requirements.

IGEM/UP/2 (the Institute of Gas Engineers and Managers installation guidance for domestic premises) is the primary technical document that engineers work to alongside BS 6891. Together these cover sizing methodology, materials, installation practice, and testing procedures for domestic and light commercial gas installations in the UK.

This article covers all aspects of domestic gas pipework — internal copper installation, CSST (corrugated stainless steel tubing), underground yellow MDPE service pipes, sizing methodology, testing procedures, and Gas Safe notification obligations.

Key Facts

• Gas Safe registration — all work on gas fittings, pipework, and appliances must be carried out by a Gas Safe registered engineer; criminal offence to work on gas without registration
• BS 6891 — British Standard for the installation and maintenance of low pressure gas pipework of up to 35mm in domestic premises; the primary standard for domestic gas
• IGEM/UP/2 — Institute of Gas Engineers and Managers document for pipework in domestic and commercial premises; includes pressure drop sizing methodology
• Copper pipe — BS EN 1057 half-hard (R220) copper tube; the standard for above-ground domestic gas pipework; 15mm, 22mm, 28mm common sizes
• CSST — Corrugated Stainless Steel Tubing; flexible gas pipe; requires earthing and bonding (see IGEM/UP/2 Issue 2); increasingly used in commercial and large residential
• Steel pipe — heavy-gauge mild steel; used in large commercial gas installations; requires anti-corrosion treatment internally and externally
• MDPE — Medium Density Polyethylene; yellow colour; used for underground gas pipework from the street main into the building
• 375mm minimum depth — minimum burial depth for MDPE service pipe in domestic gardens and footpaths
• 600mm under roads — minimum burial depth for gas pipe under a road or driveway subject to vehicular traffic
• 100mbar working pressure — standard domestic gas working pressure (LP Natural Gas in the UK); test pressure is typically 100mbar or as specified
• Let-by test — low-pressure tightness test on the appliance side of a manual valve; confirms no gas passing the valve seat; mandatory before any work on live gas
• 28-day notification — Gas Safe engineer must notify Gas Safe within 28 days of completing gas work; certificate issued to customer

Quick Reference Table

Detailed Guidance

Material Selection: Copper vs CSST vs Steel

Copper half-hard (R220): This is the standard material for domestic internal gas pipework. It is available in straight lengths (3m and 6m) and can be bent with a pipe bender. Jointing is by:

• Compression fittings (BS EN 1254-2): suitable for accessible joints; can be undone and remade; use approved olives (copper or brass)
• Capillary solder fittings (BS EN 1254-1, lead-free solder): preferred for concealed work; permanent; require flux cleaning after soldering
• End-feed (solderable) fittings: heat applied to solder each joint individually
• Yorkshire/integral solder ring fittings: solder pre-loaded in the fitting

Do not use pushfit fittings (speedfit, JG Speedfit) for gas pipework — they are not approved for gas service.

CSST (Corrugated Stainless Steel Tubing): CSST (Gastite, Tracpipe) is a flexible corrugated tube in a yellow polymer jacket. It threads through voids and around obstacles more quickly than rigid copper. However:

• CSST requires bonding to earth at each appliance connection — the corrugated tube is more susceptible to lightning-induced surge damage than rigid pipe
• Clipping is required at closer centres than copper: typically every 600mm horizontal, every 1.5m vertical
• Fittings are proprietary (Gastite or Tracpipe) — do not mix fitting brands
• CSST must be fully enclosed in conduit or trunking where it passes through walls or floors
• CSST is not suitable for outdoor above-ground use without protection

Steel pipework: Heavy gauge mild steel pipe (black steel, internally painted yellow for identification) is used for large commercial installations and high-pressure distribution. Threaded steel joints require PTFE tape approved for gas use (yellow gas-grade PTFE, not white plumbing PTFE).

Underground MDPE Installation

Yellow MDPE (Medium Density Polyethylene, BS EN 1555) is the standard for underground gas service pipes:

Minimum sizes:

• Domestic service: 25mm MDPE minimum (allows for future demand increase and pressure drop over long runs)
• Larger properties or commercial services: 32mm, 50mm, or larger

Burial depths:

Bedding and cover:

• 50mm sand or fine material bedding under the pipe
• 50mm sand or fine material cover over the pipe
• Yellow marker tape positioned 200mm above the pipe during backfill
• Backfill compacted in 150mm layers

Above/below ground transitions: At the point where the service pipe rises from underground and enters the building, a transition fitting connects the MDPE to the above-ground copper or steel pipe:

• PE to steel transition fittings (electrofusion or mechanical) are used
• The MDPE must terminate at or below ground level; steel or copper is used for the above-ground portion
• A sleeve/casing must be installed where the pipe passes through the wall — the pipe must not be in direct contact with the masonry
• The annular space between the sleeve and pipe must be sealed with a non-setting gas-resistant compound

Pipe Sizing: IGEM/UP/2 Method

Gas pipe sizing uses pressure drop calculations based on the total gas demand of all appliances connected:

Step 1: Calculate total gas demand Sum the rated inputs of all gas appliances in kW:

• Gas cooker: 12–18 kW
• Gas hob (4 burners): 10–12 kW
• Combination boiler: 24–35 kW
• Gas fire: 4–8 kW

Step 2: Apply diversity Not all appliances run simultaneously. IGEM/UP/2 provides diversity factors (Table 1 in the document):

• First appliance: 100%
• Second appliance: 80%
• Third appliance: 60%
• Each subsequent appliance: 25%

Step 3: Calculate pressure drop Using IGEM/UP/2 Table 5 (or approved software), find the pipe size that keeps pressure drop below 1mbar for each pipe run from the gas meter to the appliance. Maximum allowable pressure drop: 1mbar total across the entire installation.

Step 4: Check velocity Gas velocity must not exceed 25 m/s in any pipe section to prevent noise and erosion.

Quick reference sizing (natural gas at 20mbar inlet pressure, 1mbar max drop, 10m run):

For runs longer than 10m, increase pipe size by one step for every 10–15m of additional length.

Pressure Testing Procedure

All new gas pipework must be pressure tested before connection to the gas supply:

Working pressure test (tightness test):

1. Isolate the installation from the gas supply
2. Apply a test pressure of 100mbar using a manometer (U-gauge or digital gauge)
3. Wait 1 minute for temperature stabilisation
4. Record the pressure; maintain for 1 minute minimum (or as specified by BS 6891)
5. No pressure drop permitted — any drop indicates a leak

Let-by test (valve seat tightness):

1. With the manual emergency control valve (ECV) closed, apply gas pressure at the inlet
2. Monitor the pressure downstream of the closed valve
3. No pressure rise on the downstream side confirms the valve seat is gas-tight
4. A pressure rise indicates valve seat leakage — the valve must be replaced

Leak detection:

• Gas leak detection fluid (leak spray) or soapy water applied to all joints
• Bubbles confirm a leak; check every visible joint
• Do not use naked flames to test for gas leaks

Gas Safe Notification

After completing gas work:

• Gas Safe registered engineers must notify Gas Safe of all work involving gas appliances and pipework
• Notification must be made within 28 days of completion
• Gas Safe issues a Gas Safety Certificate or a Landlord Gas Safety Record (for rental properties)
• Landlord Gas Safety Records must be renewed annually

Penalties for working on gas without Gas Safe registration: up to 6 months' imprisonment and/or unlimited fine under the Gas Safety (Installation and Use) Regulations 1998.

Internal Pipework Identification

Above-ground internal gas pipework must be identifiable:

• Copper gas pipe is not required to be painted; its copper colour is distinctive
• Steel gas pipe must be painted yellow (chrome yellow, BS 4800 colour 08 E 51) at intervals throughout its length, and adjacent to every valve, meter, and fitting
• Concealed pipework must have identification markers at inspection points

Frequently Asked Questions

Can I use plastic push-fit fittings on gas pipes?

No. Push-fit fittings (Speedfit, JG Speedfit, Hep₂O) are approved for water only and must never be used on gas pipework. The only approved jointing methods for domestic gas are: capillary solder fittings, approved compression fittings, press fittings (Viega, Geberit with gas approval), or proprietary CSST fittings. Always check the fitting manufacturer's approval documentation before use.

Does gas pipework need to be bonded?

Main equipotential bonding must connect the incoming gas service pipe to the main earth terminal. This is a single bond (at least 10mm² copper conductor) at the point where the gas service pipe enters the building. CSST (Corrugated Stainless Steel Tubing) requires additional supplementary bonding at each appliance connection because of its susceptibility to lightning-induced surge.

What happens if the pressure test fails?

If the tightness test shows a pressure drop, the leak must be located and rectified before any further work. Use leak detection fluid on all joints. If the leak cannot be found by visual inspection, a more sensitive leak detection device (electronic gas detector) may be needed. Once repaired, the test must be repeated from the beginning. Do not connect the installation to the gas supply until the test passes with zero pressure drop.

Can I bury copper pipe underground?

Copper is generally not appropriate for direct burial underground. Copper is susceptible to corrosion in aggressive soil conditions (acidic soils, soils containing sulphur or chlorides, soils contaminated with salts). Where copper must pass underground (e.g. short sleeve through a wall foundation), it must be sleeved in a continuous protective sleeve and sealed at each end to prevent moisture contact. For all genuine underground runs, yellow MDPE is the correct material.

What size meter is needed for my gas installation?

The gas meter must be sized to pass the maximum instantaneous demand of all installed appliances. For domestic installations:

• U6 meter (standard domestic): 6 m³/h capacity — suitable for most homes with boiler, fire, and cooker
• U16 meter: 16 m³/h — required for larger homes with multiple appliances
• U25 meter: 25 m³/h — small commercial The DNO (gas network operator) is responsible for meter sizing and installation; the Gas Safe engineer specifies the required capacity.

Regulations & Standards

• Gas Safety (Installation and Use) Regulations 1998 (SI 1998/2451) — fundamental gas safety legislation; requires Gas Safe registration

• BS 6891:2005+A2:2008 — installation and maintenance of low pressure gas pipework up to 35mm; primary domestic standard

• IGEM/UP/2 — Institute of Gas Engineers and Managers; pipework design and installation guidance; sizing methodology

• BS EN 1057:2006+A1:2010 — copper tubes; specification for copper tubes for water, gas, and other applications

• BS EN 1555 — plastics piping systems for supply of gaseous fuels; MDPE specification

• Gas Safe Register Technical Standards — Gas Safe Register

• IGEM/UP/2: Installation Pipework — Institute of Gas Engineers and Managers

• HSE: Gas Safety Regulations — Health and Safety Executive

• BSI BS 6891 — British Standards Institution

• CORGI Technical Library — technical guidance from CORGI

• flue liner installation — flexible stainless steel flue liner installation

• interlock systems — commercial kitchen gas interlock systems

• pipe materials — copper, plastic, and MDPE pipe comparison